Photographic diffusion transfer process



Feb. 6, 1962 E. c. YACKEL ETAL 3,020,155

PHOTOGRAPHIC DIFFUSION TRANSFER PROCESS Filed May 25, 1956 EXPOSURE l3 S/LVER HAL/DE -AL/(AL/ SOLUBLE /CELLULOSE DERIVATIVE VEH/OLE S/L VER PREO/P/ TA T /NG LAYER DE VELOPMENL' D/FFUS/ON TRANSFER OF RES/DUAL 115 SILVER HAL/DE AND REMOVAL OF EMULS/ON LAYER/2 Stage 2 l4 a'mmwm ARGE N TAL IMAGE SUPPORT ARTHUR A. RASOH DONALD P. FOSTER EDWARD 6. VAC/(EL HENRY C. YUTZY ATTORNE 8 AGENT "tent $326,155 Fatentecl Feb. 6, 1962 ice ration of New Eersey Filed May 23, 1956, Ser. No. 585,705 11 (Ilaims. (Cl. 96-29) This invention relates to a photographic process of the silver halide difiusion transfer type wherein undeveloped silver halide of an emulsion layer is transferred as a silver complex imagewise by imbibition to a silver precipitating or nucleating layer generally to form a positive image therein.

As is known in the art,'this type of process can be carried out by developing an exposed emulsion layer with a developing solution containing a silver halide solvent after which the emulsion layer is placed into contact with a second element carrying a silver precipitating layer to cause the undeveloped silver halide to transfer imagewise as a silver complex to the second element and form an image thereon.

The process has also been carried out utilizing a multilayer element such as one carrying on a support two gelatin layers of diiierent solubility, for example, the layer next to the support containing a silver precipitating agent and being harder than the outer gelatin layer which is a gelatin emulsion layer containing silver halide. After exposure of the element, development is carried out with the silver halide solvent-containing developer to obtain a silver positive in the hardened gelatin silver precipitating layer, after which the emulsion layer containing the negative silver image is washed off, leaving the positive silver image on the support. As a practical matter, it is diffioult'to harden the underlying gelatin layer of an element of this type without unduly hardening the outer gelatin emulsion layer and even then over a period of time the emulsion. layer tends to harden which makes its diiferential removal in the final. step of the process diflicult or impossible.

In order to overcome these and other difiiculties, we have attempted to prepare various multilayer elements of the type indicated above in which the gelatin of the emulsion layer is replaced wholly or for the greater part by other organic colloid vehicles having a decidedly different solubility than the organic colloid vehicle present in the silver precipitating layer. Hydrophilic synthetic polymers such as partially hydrolyzed polyvinyl esters, polyvinyl acetals, polyamides, polyvinyl alcohol, polyvinyl pyrrolidone and non-carboxylated cellulose derivatives such as methyl cellulose, are not satisfactory for use as the emulsion vehicle since they are not readily soluble in water. These polymeric materials are generally considered to 'be soluble in water, but for the purpose of our invention they do not possess the desired physical properties. When it is attempted to remove emulsions containing a major proportion of these vehicles, from the silver precipitating layer, the emulsion is either dissolved very slowly, requires rubbing for removal, is incompletely removed or forms scum which may deposit on the processing equipment and clog drainage facilities.

We have discovered that by far the most satisfactory organic colloid materials for use as the emu-lsionvehicle of the multilayer element of the type described are alkalisoluble acid-insoluble carboxylated cellulose derivatives such as the alkali-soluble acid-insoluble dibasic acid halfesters of cellulose ethyl ethers; e.g., the cellulose ether phthalates or the alkali-soluble acid-insoluble dibasic acid half-esters of cellulose acetate, e.g., cellulose acetate phthalates; When these materials are employed as vehi- 2 cles for emulsions superposed on silver precipitating layers of a diflferent solubility, the emulsion is quickly removed with aqueous alkaline solutions. That is, after the development step is complete and the soluble silver com.- plex has migrated ima-gewise to the silver precipitating layer, the carboxylated cellulose derivative is present as the alkali metal, ammonium, or amine salt, which is highly soluble in water although not soluble in the alkaline developer because of its high salt content. Therefore, when the developed element is subsequently washed with water, the emulsion disintegrates with surprising rapidity and the positive image which remains on the silver precipitating layer is quickly freed of any contaminating materials by a brief wash with water.

The acid-insoluble characteristic of the above carboxylated cellulose derivatives is likewise important due to the fact that the coating of these derivatives is greatly facilitated by this characteristic. That is, since the carboxylatecl cellulose derivatives are insoluble in acid, coatings thereof may be caused to quickly set'during high speed coating operations by applying the coating to an acidic surface which in the present invention takes the form of a support having on its surface or in the silver precipitating layer an acidic material such as an organic, acid, e.g., citric acid. The acid causes the carboxylated cellulose derivative coating to set, and subsequent drying operations can be carried out without undue distortion of the coating. Materials such as methyl cellulose being soluble in both aqueous alkali and aqueous acid .do not possess the mentioned characteristic.

The objects of our invention therefore include providing multilayer elements useful in the silver halide diffusion transfer process. Other objects reside in providing processes for using the elements for obtaining reproductions by the silver halide diffusion transfer process.

a In the accompanying drawing is illustrated in greatly enlarged cross-sectional view a sensitive element comprising a part of our invention, as well as the general method for obtaining photographic reproductions by our silver halide diifusion transfer process. As shown therein, in Stage 1 a representative sensitive element includes a support 10 such as paper, cellulose ester or synthetic resin, if desired carrying a subbing layer not shown, and on the support the silver precipitating layer 11, for example, a gelatin layer containing a colloidal heavy metalor heavy metal sulfide and thereon the emulsion layer 12 containing silver halide grains dispersed in the alkalisoluble acid-insoluble cellulose derivative. As shown in Stage {1, the element is exposed to a line halft'one or continuous-tone subject in a suitable manner, preferably so as to obtain a right-reading reproduction, the exposure being shown as taking place in area 13 of emulsion layer 12.

In carrying out the process of our invention, the exposed element is then developed with a silver halide developing solution containing a silver halide solvent such as sodium thiosulphate with the result that a silver image is developed in area 13, and shortly thereafter the residual undeveloped silver halide is transformed to a soluble silver complex a portion of which. diffuses imagewise to the silver precipitating layer 11 where an argental image is formed by reaction of the silver complex with the silver precipitating agent of the silver precipitating layer., The emulsion layer 12' containing the negative silver image is then merely washed-from the support with an excess of water to obtain the reproduction shown in Stage 2 of the drawing wherein the argental positive image occupies-areas 14 of the silver precipitat- .herein is a water-permeable organic colloid layer containing 21 silver precipitating agent, that is, an agent cap able of yielding with silver ion a dark-colored argental or silver-containing substance, when dissolved silver salts from the emulsion layer come into contact with it. The silver precipitating agent may comprise either physical development nuclei or a chemical precipitant for silver ions.

Suitable silver precipitating agents for use in the silver precipitating layer for forming the argental image include sufides, selenides, polysulfide-s, polyselenides, thiourea, mercaptans, stannous halides, heavy metals and heavy metal salts, and fogged silver halide. Heavy metal sulfides such as lead, silver, zinc, antimony, cadmium and bismuth sulfides are useful, particularly the sulfides of lead and zinc alone or in admixture, or complex salts of these with thioacetamide, dithio-oxamide, or dithiobiuret. The heavy metals include silver, gold, platinum, palladium and mercury preferably in the colloidal form. The noble metals are particularly efficacious.

The silver precipitating agents may be applied directly to a support such as papenor to suitably subbed cellulose derivative supports and synthetic polymer supports from solutions or dispersion of the silver precipitating agents in a colloid vehicle such as gelatin. The colloid vehicle of the silver precipitating layer is necessarily insoluble in the solution used for removing the carboxylated cellulose derivative emulsion layer in the final stage of the process. If gelatin is used, it should be hardened. Hydrophilic cellulose esters and synthetic polymers are useful as a vehicle in the silver precipitating layer insorfar as they meet the requirement of having a solubility appreciably different fro mthat of the colloid vehicle of the emulsion layer.

The silver halide emulsion can be applied directly over the silver precipitating layer, but preferably a thin layer of one of the alkali-soluble carboxylated cellulose derivatives, e.g., a dibasic acid ester of the cellulose ethyl others is first applied followed by the emulsion layer. This thin layer is not absolutely essential; however, it has been found to facilitate the clean removal of the emulsion from the silver precipitating layer.

The silver halide component of the mentioned emulsions is not especially critical and can include various silver halides and mixtures of silver halides such as silver bromoiodide, silver chloride or silver bromide optically sensitized in the usual manner.. The emulsions may be developingout emulsions designed for development to negative images in which case the image obtained in the silver precipitating layer is a positive With respect to the original subject. If desired, the emulsion can be of the direct positive type with the result that the silver image developed in the emulsion is a positive and the image in the silver precipitating layer a negative in respect to the original subject. The procedures of Leermakers U.S. Patent 2,184,013 and Kendall et al. U.S. Patent 2,541,472 are useful in conferring the direct positive characteristics to the emulsions.

When the direct-positive emulsions are used in the process of the invention it is sometimes advantageous to dispense with or reduce the concentration of the sulfurcontaiuing compounds, such as the azole compounds men tioned hereinafter, when it is actually desired to obtain sepia or warm-toned reproductions. When our invention is practiced in this manner the portrait photographer can readily prepare photographs having the desired tone.

As mentioned, the colloid vehicle of the silver halide emulsion applied to the silver precipitating layer is an alkali-soluble acid-insoluble carboxylated cellulose derivative such as a dibasic acid half-ester of a cellulose ethyl ether; however, as shown in the examples hereinafter, a portion of the colloid vehicle may be some other watersoluble organic colloid such as unhardened gelatin in minor quantity and of a kind such that the removal of the emulsion layer is not adversely affected in the later stages of the process.

Representative carboxylated cellulose derivatives are the alkali-soluble acid-insoluble dibasic acid esters of the cellulose ethyl others including the phthalic, succinic, and maleic acid esters of etheyl celluloses and their ammonium, alkali metal and amine salts, the esters being made rom cellulose ethyl ethers having an alkoXyl content of at least 42 percent and the esters having a dicarboxylic acid radical content of at least 5 percent and preferably at least 20 percent.

The ethyl cellulose phthalates made from cellulose ethyl others having at least 42 percent ethoxyl and containing at least 5 and preferably about 20 percent phthalyl are particularly efficacious for use in the emulsion layer of the sensitive elements of our invention. Thus a satisfactory cellulose ether pht'nalate can be made by the esterification of a cellulose ethyl ether containing 42% ethoxyl until about 5 to 1()% phthalation has. taken place. A preferred cellulose ether phthalate is thus made from a cellulose ethyl ether containing 45% cthoxyl, the final ester containing about 24% phthalyl. The viscosity of this ester was about 3 to 6 cps. in a 3% solution containing 70% ethyl alcohol, 20% isopropyl alcohol and 10% butanol. The ethyl cellulose phthalates employed may vary also as to viscosity. We have found that in the case of low viscosity esters, such as those whose salts have a viscosity of less than 10 cps. in 4 percent solution in Water, it may be desirable to incorporate some plasticizer, such as triacetin or polyethyleneglycol in the ethyl cellulose phthalate composition.

The preparation and properties of these ether phthalates and methods for preparing emulsions containing them, suitable for application over the silver precipitating layer of our sensitive elements, is described more fully in the Talbot and McCleary U.S. Patent 2,725,293, granted November 29, 1955. In addition to the peptizing agents disclosed in the last-mentioned invention for preparing the cellulose ether phthalate emulsions, gelatin is equally useful as shown in the examples hereinafter. vMalrn et. al. U.S. Patent 2,718,667 and Hiatt et al. U.S. patent application Serial No. 272,697, filed February 20, 1952, now U.S. Patent 2,759,925, may also be referred to for a description and synthesis of useful cellulose ether phtha lates.

The alkali-soluble acid-insoluble dibasic acid esters of cellulose acetate such as various cellulose acetate phthalates are likewise useful as the emulsion vehicle of the invention. A typical cellulose acetate phthalate contains 34% phthalyl and 19% acetyl. These esters can be made by methods known in the art or as shown in the above Hiatt et al. invention. Similarly, cellulose phthalate containing about 50% phthalyl can be used as the major part of the emulsion vehicle.

The silver halide developing solution used for initiating development of the exposed sensitive element described is not especially critical and can be of the conventional type used for developing films or papers with the exception that a silver halide solvent such as sodium thiosulphate, sodium thiocyanate or ammonia is present in the quantity required to form the soluble silver complex which diffuses imagewise to the silver precipitating layer. Silver halide developing agents useful in the developing solution include hydroquinone, monomethyl-paminophenol sulfate, aminophenols, halogenated hydroquinones, toluquinone, p-hydroxyphenyl aminoacetic acid, S-pyrazolidone developing agents such as l-phenyl-S- pyrazolidone and mixtures of these developing agents. Useful developer compositions containing combinations of 3-pyrazolidone silver halide developing agents with weak developing agents such as ascorbic acid are disclosed in U.S. Patents 2,688,549, 2,691,589, 2,688,548, 2,685,515, 2,685,516 and in the James et al. U.S. patent application Serial No. 443,536, filed July 15, 1954, now U.S. Patent2,75 1,300 granted June 19, 1956.

The image tone and tonal range of the reproductions obtained in our process are measurably improved if development of the sensitive element and the imagewis'e diffuaoaaies sion of the silver complex takes place in the presence of certain sulfur-containing azole compounds, particularly the mercapto-1,3,4-oxadiazoles and mercapto-1,3,4-thiadiazoles, e.g., Z-rnercapto-S-phenyl-1,3,4-oxadiazole J.C.S. 4813 1952) (5 (Z-met-hoxyphenyl) 2 mercapto 1,3,4 oxadiazole, and Z-amino-S-mercapto-1,3,4-thiadiazole (J.A.C.S. 52 4860 (1930)). The latter compound is particularly effective in obtaining black reproductions when employed with an element such as described having silver bromoiodide (or other silver halide) in the removable layer overlying the silver preciptating layer. For this purpose good results are obtained when about 0.0002 to .003 gram of the thiadiazole compound are present per square foot of surface whether in the emulsion layer initially, in the silver precipitating layer, or is provided by the developer solution. About 0.20 gram per liter is a satisfactory amount to use in the developers containing silver halide solvent. The compound has been found to be eifective in producing black toned images with silver bromoiodide emulsions where other sulfur compounds have failed. To obtain the desirable efiects from these compounds, they can be employed in either the emulsion, the silver precipitating layer or some other layer adjacent to or not too remote from the emulsion layer, or in the developer solution particularly in the case of the mentioned oxadiazole compounds. The benzothiazoline-Zdhiones, e.g., 3-methylbenzothiozoline-2-thione,

N C H:

(III) and 3-carboxymethylbenzothiazoline-Z-thione can also be advantageously present during development. These compounds are especially useful when employed in the emulsion layer although they may be used in the silver precipitating layer or other layer of the sensitive element or in the developing solution with similar results. The concentration of the sulfur-containing azole compounds in the silver halide solvent containing developer solution and in the sensitive element is not especially critical. For example, the oxadiazole compound mentioned can be present in the developing solution in a concentration of the order of 0.20 to 2 grams per liter of solution. In general, an increase in the concentration of the sulfur-containing compounds mentioned results in a decrease in contrast of the final image which is very desirable since the diffusion transfer processes including ours herein described tend to produce high contrast images. This means of controlling contrast is naturally of less importance when making line or halftonereproductions than when making reproductions from continuous tone originals.

The tone-modifying properties of the mentioned sulfurcontaining azole compounds, e.g., 2-mercapto-5-phenyl- 1,3,4- oxadiazole I, 2-amino-5-mercapto-l,3,4-thiadiazole II and 3-methylbenzothiazoline-Z-thione III can best be taken advantage of when the compounds are employed during the development step in combination witheach other or with l-phenyl-S-mercapto-1,2,3,4-tetrazole IV.

For example, it has been ascertained that when compounds I, III and IV are employed alone in an emulsion layer of an element such as shown in the drawing and prepared as described in Example 1 hereinafter, approximately 0.4, 2.0 and 0.2 gram respectively of I, III and IV per mole of silver halide are required to obtain neutral toned reproductions of maximum density. However, when the compounds are employed in the emulsion in combination much less of either compound is required to obtain the same result. Thus combinations of 0.04 gram of IV and 1.0 gram of III, or 0.1 gram of IV and 0.2 gram of I, or 0.4 gram of I and 0.8 gram of III (per mole of silver halide) yield neutral toned high density images in the process.

In addition to improvement in tone, the compounds in combination produce valuable improvements in contrast of the image. For example, Compound IV used alone produces contrasts as high as 4.4 to 4.7 which is typical of the silver halide diffusion transfer processes previously known. When IV and III are used in combination in optimum proportions a contrast of 2,6 or less can be obtained, and the combination of I and III readily gives a contrast as low as 1.5. Similar results are obtained when the compounds are used in combination in either the emulsion, developer or silver precipitating layer of the element. Useful results are obtained when the combinations of the compounds are used in admixture or are distributed throughout the sensitive element or developer, i.e.. one compound may be in the developer and the other in the sensitive element.

A further tone-modifying compound which is it atvantageous to have present during the development of the described element having the removable emulsion layer coated over the silver precipitating layer, is xylene sulfonate (l,3-dimethylbenzene-4-sulionic acid). The compound is useful particularly as the alkali metal salt in the developer, emulsion or silver precipitating layers of the elements described. For example, the following developer solution containing silver halide solvent yields neutral toned images with little effect upon the shape of the sensitometric curve.

Grams Monomethyl-p-aminophenol sulfate 3.6 Hydroquinone 18.0 Sodium sulfite 43.0 Sodium thiosulphate 24.0 Sodium hydroxide 1.8.0 Benzotriazole 0.09 Xylene sulfonate (Na salt) 100.0

Water to make 1 liter.

While the mentioned mercapto-l,3,4-oxadiazoles, mercapto-1,3,4-thiadiazoles, benzothiazoline-2-thiones and xylene sulfonate are particularly efiicacious for use in our invention as described, in conjunction with the two layer element of the drawing, for controlling image tone and tonal range, these compounds are useful in the same manner alone or in combination with each other or with l-phenyl-5-rnercapto-l,2,3,4-tetrazole in any of the silver halide diffusion transfer processes in which a silver halide image is transferred imagewise to a receiving layer as a silver salt complex in the presence of a silver halide solvent and a silver precipitating agent and an argental image is for-rnedtherein. Thus, in a well known diffusion transfer process wherein an exposed silver halide emulsion layer is developed then placed in contact with a silver precipitating stratum in the presence of a silver halide solvent, the above compounds are advantageously present alone or in combination during either or both the development step and the transfer step. In that type of process xylene sulfonate may be expected not only to yield neutral toned images but can be used to control the shape of the characteristic curve of the image, i.e. to improve highlight gradation by extending the toe region. A representative developer composition useful for devcloping an exposed emulsion layer in contact with a silver precipitating stratum is as follosw:

Grams Hydroquinone 35.0 Sodium sulfite 80.0 Sodium hydroxide 28.5 Sodium thiosulphate 15.0 Xylene sulfonate (Na salt) 25.0

Water to make 1 liter.

The amount of xylene sulfonate in the developer can obviously be varied depending upon the results desired with the particular emulsion in use. Amounts from to 100 grams per liter of developer solution will give useful results. Similarly, various amounts of the sulfonate can be used in the emulsion or silver precipitating stratum in this type of diffusion transfer process.

A process in which the mentioned sulfur-containing azole compounds or xylene sulfonate are particularly useful alone or in combination with each other, is disclosed in the Yutzy et al. U.S. patent application Serial No. 543,886, filed October 31, 1955, now U.S. Patent 2,882,151. In that process an element is provided having a gelatino-silver halide emulsion layer on one side of an opaque support and a silver precipitating stratum on the other side of the support. After exposure of the emulsion, e.g., in a camera, development iscommenced, then the element is rolled up with the emulsion innermost to allow the emulsion layer to come into contact with the silver precipitating stratum in the presence of the mentioned azole compounds present in the emulsion, silver precipitating stratum or developer solution. The element is kept rolled up until the negative image has developed in the emulsion layer and the residual silver halide has been dissolved by the silver halide solvent in the developer and transferred imagewise as a complex silver salt to the silver precipitating stratum. The element is then unrolled to reveal a positive argental image on the support side of the element, the negative image of course being Visible from the emulsion side.

In this process and the processes of the examples hereinafter, the silver halide developing agent may be present in either the developing solution or the emulsion itself. In the latter case it is preferable to use a substantially non-diffusing developing agent such as 3,4-dihydroxydiphenyl in the emulsion layer.

Our invention will now be illustrated in more detail by the following examples.

Example 1 A gelatin solution (250 cc.) was diluted with 750 cc. water. 2.5 cc. of 1 N Na S were then added to the solution. 250 cc. of Water containing 2.6 cc. of 1 N zinc nitrate were added slowly to the sulfide solution through a jet, resulting in the formation of a colloidal dispersion of zinc sulfide. To this dispersion were added 10 liters of a 3% gelatin solution, 300 cc. of a 7.6% saponin solution and 134 cc. of a 10% formaldehyde solution. The mixture was dispersed at 40 C. and then coated on a photographic paper support at a coverage of 2 lb. of solution per 100 sq. ft. of coated surface and dried, forming the silver precipitating layer.

(Ever the silver precipitating layer was coated a 1% aqueous solution of the sodium salt (or other alkali metal salt) of cellulose ether phthalate (an ethyl cellulose containing 45.8% ethoxyl phthalated to 22.7% phthalyl) at a coverage of 0.15 lb. per sq. ft.

To 1 liter of a positive speed, sulfur sensitized, high contrast chlorobromide emulsion containing approximately 30 g. of gelatin and one mole of silver halide per liter, were added the following ingredients: (Such emulsions containing low gelatin content can be prepared by the procedures described in the MacWilliam U.S. patent application Serial No. 440,282, filed June 29, 1954, and

8 the Yutzy and Frame U.S. Patent 2,614,928, granted October 21, 1952).

30 cc. of a 50% aqueous solution of glycerine 20 cc. of a solution containing 34 gm. of salicylaldehyde oxime per liter of MeOH 30 cc. of a 7.6% saponin solution 12 /2 cc. of a solution containing .2 gm. of l-carboxymethyl 5 [(3 ethyl-2(3)-benzoxazolylidene)-ethylidene]-3-phenyl-2-thiohydantoin in 30 cc. of MeOH+l drop of triethylarnine To the above mixture were added 4 liters of a 4% solution of the ammonium salt of the cellulose ether phthalate just mentioned and 2 gm. of 3-methylbenzothiazoline-Z-thione dissolved in methyl alcohol. The mass was stirred at 30 C., thoroughly dispersed and then coated over the sodium salt of cellulose ether phthalate interlayer at a coverage of 600 sq. ft. per mole of silver halide and dried in the conventional manner.

After exposure, the direct positive material was exposed and processed for one minute in the following developer at 70 C.

Water1000 cc.

Sodium sulfite (des.)60 gms.

Sodium hydroxide-25 gms.

Potassium bromide2 gms.

1% benzotriazole (in water)12 /z cc.

Sodium thiosulfate (1 gm. in 3 cc. Water)- cc.

25 gms. hydroquinone dissolved in 100 cc. of methanol Monomethyhp-aminophenol sulfate-5 gms.

The processed paper Was washed for 20-30 seconds in cool water (30 C.) to remove the emulsion layer, leaving only the transferred positive image in the nucleating layer. The material was then rinsed in a 5% acetic acid stop bath for 5 seconds and dried.

The silver precipitating layer of the element just described can be prepared in a somewhat different manner as follows:

250 cc. of a 10% gelatin solution were dispersed in 750 cc. water. To this solution were then added 2.5 cc. of a 1 N sodium sulfide solution. To this mixture there were added 250 cc. of Water containing 1.67 cc. of a 1 N zinc nitrate solution. The dilute zinc nitrate solution Was added slowly to the sulfide solution through a jet, resulting in the formation of a colloidal dispersion of zinc sulfide in the presence of excess sulfide ions. To this dispersion were added 10 liters of a 3% gelatin solution, 300 cc. of a 7.6% saponin solution and 134 cc. of a 10% formaldehyde solution. The mixture was dispersed at 40 C. and then coated on a photographic paper support at a coverage of 2 lbs. of solution per 100 ft. of coated surface and dried.

Example 2 A layer of gelatin containing colloidal silver of the type described by Carey-Lea was coated on paper base sov that the quantity of metallic silver per square foot was approximately 1.5 mg. and gelatin per square foot approximately 550 mg. This layer was hardened by the addition of 10% formaldehyde (1 part) to 5% gelatin parts) before coating. After the layer was dried it was overcoated with a silver halide emulsion dispersed in a cellulose ether phthalate, similar in composition to that used in Example 1. The emulsion was coated so that approximately 0.3 g. silver halide and 0.3 g. cellulose ether phthalate were laid down per square foot. After exposure to a positive, the layer was developed for 30 seconds in the following solution:

Water cc 200.0

9 During this development, a negative silver image formed in the cellulose ether phthalate layer and a positive image in the nucleated gelatin layer. After the development the sheet was treated with tap water at a temperature of approximately 100 F, whereupon the cellulose ether phthalate emulsion layer dissolved and was very easily and cleanly removed leaving a direct positive image of the original.

Example 3 The improvement in image tone in a diffusion transfer process using the mentioned azole compounds is illustrated herein:

A silver bromoiodide negative emulsion was exposed and immersed for 6 seconds in a developer consisting of:

Grams Hydroquinone 32.8 Sodium sulfite (anhydrous) 75 Sodium thiosulphate 14 Sodium hydroxide 26.7 Benzotriazole 0.12 Water to make 1 liter.

Immediately after removal from the developer, the negative sheet was brought in contact with the following receiving sheet in a superposed position by running the two sheets through a pair of wringer rollers. After 2 minutes in contact, the two sheets were stripped apart to obtain a black positive image on the receiving sheet.

The receiving sheet for this example was prepared in the following manner. A solution comprising Water 120 0.5 M sodium sulfide 1.8 0.5 M sodium silicate a 6 was mixed rapidly at 40 C. with a solution comprising Water 390 0.5 M lead acetate 1.8

0.5 M zinc nitrate 0.72 0.1 M silver nitrate 1.8

Dilute nitric acid to pH 3.2.

After 2 minutes of stirring 90 cc. of a 20 percent gelatin solution was added, and after 30 more minutes of stirring, the pH was adjusted to 3.0 with dilute nitric acid. At this point 9 cc. of a 0.5 percent solution of 2-amino-5- mercapto-1,3,4-thiadiazole was added, together with spreading and hardening agents. The coating Was made 7 at 2 cc./ft. on film base.

Example 4 When the process of Example 3 is carried out omitting the diazole compound from the receiving sheet the following developing solution is suitable Grams Hydroquinone 35 Sodium sulfite 80 Sodium hydroxide 28.5 Sodium thiosulphate l Z-mercapto-5-amino-1,3,4-thiadiazole 0.20 Water to make 1 liter.

Example 5 The process of Example 3 was carried out except using a silver bromoiodide emulsioncontaining 0.09 gram silver halide (calculated as silver), 0.5 gram of gelatin and 0.0012 gram of the azole compound per sq. ft. of emulsion.

10 Example 6 A suitable develo er to use in place of that shown in Example 1 is as follows:

Grams Hydro quinone 1 8 Monomethyl-paminophenol sulfate 3.6 Sodium sulfite 43 Sodium thiosulrate 24 Sodium hydroxide 18 2-mercapto-5-amino-1,3,4-thiadiazole 0.20 Water to make 1 liter.

As a result a neutral toned image was obtained in the silver precipitating layer.

Example 7 l-phenyl-Snrercapto-l,2,3,4-tetrazole was added to the emulsion prepared as in Example 1 in varying amounts ranging from 0.02 to 1.6 grams per mole of silver halide and the process of Example 1 carried out. A black image was obtained only when the concentrations of 0.20 to 1.6 grams of the tetrazole per mole silver halide were used, contrast increased gradually from 2.6 to 4.7 with increase in concentration and the increase in maximum density was only 56% at the 0.20 gram level.

Example 8 Example 1 carried out with varying amounts of 3 methylbenzothiazoline thione in the emulsion showed that 2.0 grams of the compound per mole silver halide were required to obtain a dark brown image.

Example 10 Example 1 process was carried out using emulsions containing from 0.1 to 1.6 grams of Z-mercapto-S-phenyl- 1,3,4-oxadiazole per mole silver halide. A black image was obtained at the 0.4 to 1.6 grams levels with density increases of to 152% over the control sample free of the oxa'diazole compound.

Example 11 Example 1 process was carried out using emulsions containing 0.4 gram of the above diazole and from 0.4 to 2.4 grams of the above thione compound per mole of silver halide. A black image was obtained at the 0.8 gram level and bluish-black at the higher levels.

- Example 12 Example 13 The process of Example 1 carried out with from 0.8 to 1.2 grams of the thione compound and 0.2 to 0.8 gram of the oxadiazole compound per mole of silver halide, produced black high density images, density increases as high as over the control samples being obtained. The above-described emulsions containing a dispersion of silver halide in an alkali-soluble carb-oxylated cellulose derivative are advantageously used in another type of process (described in detail in Example 14 hereinafter) wherein it is also desired to remove the emulsion after it has served the purpose of recording the negative image. An element is provided similar in structure to that shown in Stage 1 of the drawing and including a support carrying a layer comprising a slow gelatino silver halide emulsion or a substantially light-insensitive emulsion layer such as an emulsion fogged by light or chemical treatment, or a desensitized emulsion. This layer contains a much larger amount of silver halide than is necessary when fogged silver halide is used in the silver precipitating layer in the process of Example 1. Over such emulsion layer is a removable layer of a much faster silver halide emulsion of which the colloid vehicle is a carboxylated cellulose derivative such as an alkali-soluble dicarboxylic acid ester of ethyl cellulose mentioned above. The element is exposed, developed for a limited length of time in a high energy developer, in absence of develop ment restrainers such as bromide, with the result that a negative image is obtained in the outer removable emulsion layer and a positive image in unexposed area of the lower emulsion layer. Removal of the alkali-soluble outer emulsion layer containing the negative image yields a direct positive reproduction on the support.

Example 14 A gelatino-silver chloride emulsion containing 0.0-5 mol of silver chloride in 280 grams of emulsion was coated on paper at 450 sq. ft. per mole of silver chloride. Then 50 grams of a bromoiodide emulsion containing 0.025 mol silver halide and 80 grams or less of gelatin permole of silver halide was dispersed in a 2% solution of a cellulose ether phthalate and coated over the slow silver chloride emulsion layer at 1500 sq. ft. per mole of silver halide. After exposure the element was developed in a sufitecarbonate-En-hydroquinone developer free of antifoggents followed by washing to remove the outer emulsion layer containing the negative silver image leaving a positive silver image in the emulsion layer on the support. The result obtained can be attributed to the fact that the induction period for the development of the silver halide grains in the outer emulsion layer is appreciably less than for the silver halide grains of the lower emulsion layer partly due to the time required for developer to diffuse to the lower emulsion layer and also due to restraining effects of oxidized developer and bromide ions released as development of the outer emulsion layer proceeds.

A similar element which can be processed as just described has a gelatino-silver chloride emulsion on the support (coated 450 sq. ft./mole silver halide) overcoated with a chlorobromide emulsion (27 grams of silver chlorobromide emulsion containing 10 grams gelatin per mole silver halide dispersed in 223 cc. of 1.8% solution of a cellulose ether phthalate) at a coverage of 1000 sq. ft. per mole of silver halide. Processing can be carried out as described just above except the developer preferably contains sodium hydroxide instead of carbonate.

A further element which can be-processed as described in this example includes a paper coated with the following emulsions:

247 grams of gelatino-silver chloride emulsion containing 0.05 mole of silver chloride are fogged at pH of 9.0 to 9.5 and 40 C. with 2.33 cc. of 2% formaldehyde solution for 40 minutes. The pH was then lowered to 5.5 with sulfuric acid and the emulsion coated at 520 sq. ft. per mole silver halide. The emulsion layer was then coated with the cellulose ether phthalate-bromoiodide emulsion immediately above at 1500 sq. ft. per mole silver halide. Processing can be carried out as described above preferably using a caustic alkali-hydroquinone-ascorbic acid-sodium formaldehyde bisulfite developer.

Contrary to the procedures of Examples 1 and 2, the developers used in this example should be relatively free of silver halide solvent such as hypo since the development process involved does not depend upon the transfer of silver halide from the outer emulsion layer to the underlying emulsion layer.

The mechanism of the process of Example 14 just-de- 12 scribed is similar to that of the processes of French Patent 716,428 and U.S. Patent 2,712,995. Accordingly, the processing techniques and solutions disclosed by those patents are applicable to the sensitive elements of this example. It will be noted that contrary to the method of the latter patent the sensitive element of this example provides a means for the selective removal of the negative silver image which does not depend upon differential solubility of negative and positive silver images. In our process the negative silver is simply removed when the alkali-soluble cellulose derivative emulsion is washed off.

Example 15 A paper was coated with a silver precipitating layer as in Example 1 followed by a silver halide emulsion prepared as in Example 1 except the 4% cellulose ether phthalate solution was replaced by a 4% solution of the cellulose acetate phthalate (34% phthalyl and 19% acetyl). The coating was exposed and processed as in Example 1 to obtain a direct-positive image of good quality.

Our method can also be used to produce a direct positive image in film of the lenticular type. This material has the silver precipitating or nucleating layer coated on the side of a lenticulated support opposite to that having the lenticules, the nucleating layer being overcoated with a silver halide emulsion layer which is removed during processing. A film of this type is described in the following example:

Example 16 Silver chlorobromide in 25 g. of gelatin per mole silver halide parts 1 Cellulose ether phthalate (4% aqueous solution of sodium salt) parts" 4 Water, per mole of silver halide liter 1 Tartrazine, per mole of silver halide grams 10 This film was exposed through the support to a tricolor filter imaged on the lenticules, the subject being blue, green, red, yellow, cyan and magenta step wedges. The film was then processed as described in Example 2, and showed good color separation.

The film of Example 16 can also be processed to a negative by developing in the developer of Example 2, but omitting the sodium thiosulfate. In this case the emulsion layer is not washed ofi.

What we claim is:

1. A light-sensitive photographic element comprising a support, a silver precipitating stratum on the support, and adhered to said stratum by means of a layer of an alkali-soluble acid-insoluble phthalate of any ethyl cellulose of alkoxyl content of at least 42%, a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkali-soluble acid-insoluble phthalate of an ethyl cellulose of alkoxyl content of at least 42%, the phthalyl content of said phthalates being at least 5%.

2. A light-sensitive photographic element comprising (1) a support, (2) a silver precipitating stratum on the support containing a silver precipitating agent of the class consisting of heavy metal nuclei, metal sulfide nuclei and metal selenide nuclei, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in a member of the class consisting of an alkali-soluble acid-insoluble dibasic acid ester of a cellulose ethyl ether and an alkali-soluble acid-insoluble dibasic acid ester of a cellulose acetate and containing a member of the class consisting of a mercapto-1,3,4-oxo- 13 diazole, a mercapto-l,3,4-thiadiazole, 1,3-dimethylbenzene-4-sulfonic acid, and a benzothiazoline-2-thione.

3. A light-sensitive photographic element comprising (1) a support, (2) a silver precipitating stratum on the support containing a silver precipitating agent of the class consisting of heavy metal nuclei, metal sulfide nuclei and metal selenide nuclei, and (3) adhered tosaid stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in a member of the class consisting of an alkali-soluble acid-insoluble dibasic acid ester of a cellulose ethyl ether and an alkali-soluble acid-insoluble dibasic acid ester of a cellulose acetate and containing l-phenyl-S-mercapto-l,2,3,4-tetrazole in combination with a member of the class consisting of a mercapto-1,3,4-

oxadiazole, l,3-dirnethylbenzene-4-sulfonic acid, a mercapto-l,3,4-thiad. ia zole and a benzothiazoline-Z-thione.

4. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum on the support containing silver precipitating nuclei, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in a member of the class consisting of an alkali-soluble acid-insoluble dibasic acid ester of a cellulose ethyl ether and an alkali-soluble acid-insoluble dibasic acid ester of a cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent for a time sufficient to form a silver image and an image-wise distribution of a soluble silver complex in the emulsion layer, allowing a portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipitated in said stratum by means of said nuclei, and removing the emulsion layer from said stratum, the diffusion of the silver complex being carried out in the presence of a member of the class consisting of a mercapto-1,3,4-oxadiazole, a mercapto-1,3,4-thiadiazole, a benzothiazoline-Z-thione and l,3-dimethylbenzene-4-sulfonic acid.

5. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum containing silver precipitating nuclei on the support, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkalisoluble acid-insoluble phthalate of a member of the class consisting of ethyl cellulose and cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent for a time sufiicient to form a silver image and an imagewise distribution of a soluble silver complex in the emulsion layer, allowing a portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipittated in said stratum by means of said nuclei, and removing the emulsion layer from said stratum, the developing being carried out in the presence of 3-carboxymethylbenzothiazoline-Z-thione.

6. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum containing silver precipitating nuclei on the support, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkalisoluble acid-insoluble phthalate of a member of the class consisting of ethyl cellulose and cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent for a time sutficient to form a silver image and an imagewise distribution of a soluble silver complex in the emulsion layer, allowing a' portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipitated in said stratum by means of said nuclei, and removing the emulsion layer from said stratum, the developing being carried out in the presence of S-(Z-methoxyphenyl) -2-mercapto-1,3,4-oxadiazole.

7. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum containing silver precipitating nuclei on the support, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkali-soluble acid-insoluble phthalate of a member of the class consisting of ethyl cellulose and cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent for a time sufiicient to form a silver image and an imagewise distribution of a soluble silver complex in the emulsion layer, allowing a portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipitated in said stratum by means of said nuclei, and removing the emulsion layer from said stratum, the developing being carried out in the presence of Z-methylbenzothiazoline-Z-thione.

8. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum containing silver precipitating nuclei on the support, and (3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkalisoluble acid-insoluble phthalate of a member of the class consisting of ethyl cellulose and cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent for a time sufiicient to form a silver image and an imagewise distribution of a soluble silver complex in the emulsion layer, allowing a portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipitated in said stratum by means of said nuclei and removing the emulsion layer from said stratum, the developing being carried out in the presence of Z-mercapto-S-phenyl- 1,3,4-oxadiazole.

9. A method of forming a photographic image which comprises exposing to a subject an element comprising (1) a support, (2) a silver precipitating stratum containing silver precipitating nuclei on the support and 3) adhered to said stratum a light-sensitive emulsion layer containing silver halide uniformly dispersed in an alkalisoluble acid-insoluble phthalate of a member of the class consisting of ethyl cellulose and cellulose acetate, developing a latent image in the emulsion layer with an alkaline silver halide developing solution containing a silver halide developing agent and a silver halide solvent to form a silver image and an imagewise distribution of a soluble silver complex in the emulsion layer, allowing a portion of said silver complex to diffuse imagewise to said silver precipitating stratum and the silver of said portion of silver complex to be precipitated in said stratum by means of said nuclei, and removing the emulsion layer from said stratum, the developing being carried out in the presence of Z-amino-S-mercapto-l,3,4-thiadiazole.

10. In a process for the production of photographic images in which an exposed silver halide emulsion layer is developed to produce a silver image and the residual undeveloped silver halide is caused to diffuse imagewise to a receiving stratum by means of a silver halide solvent, and an argental image is formed therein by interaction of said dilfused silver halide and a silver precipitating agent, the improvement in which said interaction takes' place in association with a member of the class consisting of a mercapto-l,3,4-oxadiazole, a mercapto-l,3,4-thiadiazole, 1,3-dimethylbenzene-4-sulfonic acid and a ben- 15 zo-thiazoline-Z-thione, to improve the tone of said argental image.

11. In a process for the production of photographic images in which an exposed silver halide emulsion layer is developed to, produce a silver image and the residual undeveloped silver halide is caused to diffuse imagewise to a receiving stratum by means of a silver halide solvent, and an argental image is formed therein by interaction of said diffused silver halide and a silver precipitating agent, the improvement in which said interaction takes place in association with a member of the class consisting of a mercapto1,3,4-oxadiazole, a mercapto- 1,3,4-thiadiazole, l,3-dimethylbenzene-4-sulfonic acid and a benzo-thiazoline-Z-tbione in combination with 1- phenyl-S-mercapto-1,2,3,4-tetrazole, to improve the tone of said argental image.

References Cited in the file of this patent UNITED STATES PATENTS 1,954,337 Staud Apr. 10, 1934 16 Schneider Aug. 28, Fallesen et al. Feb. 18, Kendall et a1. Oct. 30, Land Nov. 4, Land Sept. 29, Rott "2 Dec. 22, Yackel Aug. 3, Herz Sept. 7, Land Oct. 26, Land Jan. 4, Weyde Jan. 11, Land Feb. 15, Land .2... Mar. 22, Talbot et al. Nov. 29, Kendall et al Nov. 29, Land Dec. 6, Land Aug. 5,

way-"'2: 5:" 1 

4. A METHOD OF FORMING A PHOTOGRAPHIC IMAGE WHICH COMPRISES EXPOSING TO A SUBJECT AN ELEMENT COMPRISING (1) A SUPPORT, (2) A SILVER PRECIPITATING STRATUM ON THE SUPPORT CONTAINING SILVER PRECIPITATING NUCLEI, AND (3) ASHERED TO SAID STRATUM A LIGHT-SENSITIVE EMULSION LAYER CONTAINING SILVER HALIDE UNIFORMLY DISPERSED IN A MEMBER OF THE CLAS CONSISTING OF AN ALKALI-SOLUBLE ACID-INSOLUBLE DIBASIC ACID ESTER OF A CELLULOSE ETHYL ETHER AND AN ALKALI-SOLUBLE ACID-INSOLUBLE DIBASIC ACID ESTER OF A CELLULOSE ACETATE, DEVELOPING A LATENT IMAGE IN THE EMULSION LAYER WITH AN ALKALINE SILVER HALIDE DEVELOPING SOLUTION CONTAINING A SILVER HALIDE DEVEOPING A GENT AND A SILVER HALIDE SOLVENT FOR A TIME SUFFICIENT TO FORM A SILVER IMAGE AND AN IMAGE-WISE DISTRUBITION OF A SOLUBLE SILVER COMPLEX IN THE EMULSION LAYER, ALLOWING A PORTION OF SAID SILVER COMPLEX TO DIFFUSE IMAGEWISE TO SAID SILVER PRECIPATING STRATUM AND THE SILVER OF SAID PORTION OF SILVER COMPLEX TO BE PRECIPATEDF IN SAID STRATUM BY MEANS OF SAID NUCLEI, AND REMOVING THE EMULSION LAYER FROM SAID STRATUM, THE DIFFUSION OF THE SILVER COMPLEX BEING CARRIED OUT IN THE PRESCENCE OF A MEMBER OF THE CLASS CONSISTING OF A MERCAPTO-1,3,4-OXADIAZOLE, A MERCAPTO-1,3,4,-THIADIAZOLE, A BENZOTHIAZOLINE-2-THIONE ANS 1,3-DIMERTHYLBNZENE-4-SULFONIC ACID. 